1. Field of the Invention
The present invention relates to channel assignment method in a cellular mobile communication system.
2. Description of the Prior Art
In a mobile communication system such as a cordless telephone system, the base station is not assigned communication channels in a fixed manner, but rather, with each demand for communication, the base station performs dynamic channel assignment in which the interference signal level is checked and a channel determined to be idle (i.e., the channel is not busy in that cell and nearby cells) is chosen and used. In this assignment, there is a channel assignment method by which assignment is made without measuring the power ratio of the desired signal to the interference signal (hereinafter referred to as Carrier to Interference Ratio [CIR]) between the neighboring cells' base station and mobile station that are already using the channel to be assigned.
In this type of channel assignment method, control is often exercised over the transmission power with the purpose of effective frequency use and decreased power consumption at terminals. The transmission power control systems can be broadly divided between two systems. In one, within the range in which the received power of the desired signal is at or above a fixed value, control is exercised to limit the transmission power to the lowest possible level. This system will here be referred to as the desired signal level system. In the other system, within the range in which the received power of the desired signal is at or above a fixed value, and moreover, the CIR is at or above a required value, control is exercised to limit the transmission power to the lowest possible level. This system will here be referred to as the CIR system.
In this type of system that performs dynamic channel assignment and transmission power control, a communication using a channel judged to be idle may cause interference to communications in other cells. Particularly, when the number of cells of large transmission power is at the same level as cells with small transmission power, the probability for interference to occur (hereinafter referred to as interference probability) becomes great. The reason for this will be explained below. First, the case must be considered in which, at a particular transmission power and while a particular channel is in use, the same channel is assigned to a cell in the vicinity at the same or less transmission power. At such a time, the interference signal level at the base station and mobile station using the same channel will frequently be at the same level or less than the interference signal level of the base station and mobile station to be newly assigned. Consequently, if the CIR in a newly assigned base station and mobile station is at or above the required value, the probability that the CIR in the already busy base station and mobile station will be at or above the required value is relatively high, and the probability of interference is small.
In contrast, in a case in which the same channel is assigned at a larger transmission power, the interference signal level for the busy base station and mobile station will more commonly be greater than the interference signal level of the newly assigned base station and mobile station, and consequently, even if the CIR for the newly assigned base station and mobile station is at or above the required value, the CIR of the in use base station and mobile station will be at or below the required value and interference is likely to occur. Accordingly, when cells of large transmission power and cells of small transmission power are mixed at approximately the same proportion, even though a particular channel is in use nearby at a low transmission power, the same channel may be assigned at a large transmission power and the probability of interference is potentially great.
When interference occurs during a communication, it becomes necessary to switch the channel during the communication. When the incidence of channel switching during communication increases, not only is there a problem with increasing burden on control, there is also the problem of a greater probability for forced disconnection resulting from switching when there are no idle channels. It is therefore necessary to limit the probability of the occurrence of interference.
In a case in which control of transmission power is not carried out by the base station and is performed only by the mobile station by the desired signal level system, by measuring the CIR of the base station and mobile station to be newly assigned, the CIR in other stations using the channel to be assigned is estimated and the occurrence of interference can be prevented. This channel assignment method is explained in the following reference:
Syuji YASUDA and Seizo ONOE, "Mobile Station Transmission Power Control for Dynamic Channel Assignment in Mobile Radio Systems." The Institute of Electronics, Information and Communication Engineers. Spring National Conference. 1991. B-333
Nevertheless, the above-described methods do not take into consideration cases in which, in a mobile communication system in which a channel is assigned without estimating the CIR for a base station and mobile station in a cell in the vicinity that is already using the channel to be assigned, transmission power control is performed at both the base station and the mobile station or transmission power control is performed by the CIR system.